1. Field of the Invention
The present invention relates to a light-emitting element utilizing electroluminescence. The present invention also relates to a light-emitting device or a lighting device which includes the light-emitting element.
2. Description of the Related Art
In recent years, research and development have been actively conducted on light-emitting elements utilizing electroluminescence. In a basic structure of the light-emitting element utilizing electroluminescence, a layer containing a light-emitting substance (hereinafter, the layer is referred to as a “light-emitting layer”) is sandwiched between a pair of electrodes. By applying voltage between the pair of electrodes of the light-emitting element, light can be emitted from the light-emitting substance.
Among light-emitting elements utilizing electroluminescence, a light-emitting element in which an organic compound is used as a light-emitting substance can be formed by stacking thin films. Thus, the thickness and weight of the light-emitting element can be reduced and the area thereof can be easily increased. For that reason, the light-emitting element is expected to be used for an area light source. Further, the light-emitting element is expected to have emission efficiency higher than that of an incandescent lamp or a fluorescent lamp, and thus has attracted attention as a light-emitting element suitable for lighting equipment.
The light-emitting element can emit light of a variety of colors depending on the kind of light-emitting substance. In particular, in consideration of application to lighting, a light-emitting element which can emit white light or light of color close to white with high efficiency has been hoped for.
As a light-emitting element which can emit white light, for example, a white light-emitting element in which a plurality of light-emitting units having emission peaks in the red, green, and blue wavelength ranges are stacked has been proposed (e.g., Patent Document 1). In addition, a white light-emitting element in which two light-emitting units having emission peaks in the wavelength ranges of complementary colors (e.g., blue and yellow) are stacked has been proposed (e.g., Patent Document 2).